The instrument systems in many single engine aircraft commonly include a number of instruments such as a heading indicator, an attitude indicator, and a turn and bank indicator, each being vacuum operated. In those systems, the aircraft engine drives a vacuum pump, and the instruments described are connected by vacuum lines to the vacuum pump. While such a system is usually provided with a vacuum gauge to indicate to the pilot any failure in the vacuum system, wherein the pilot can then revert to visual flight, in some instances, such as when visibility is poor, failure of the vacuum system can be critical. In other single engine aircraft these same instrument systems are dependent on an air pressure source rather than a vacuum source. Here the aircraft engine drives an air compressor pump and air lines connect the air compressor pump to the instruments. These systems include an air pressure gauge to indicate to the pilot any failure in the instrument system, but once again if visibility is poor, failure of the system can be critical. While redundant or back-up systems have been employed in other applications such as in steering systems or fuel feed systems, no simplified or effective redundant systems of the type of the invention have been employed in the prior art.
Examples of the prior art back-up system for steering or fuel supply applications are illustrated in the Lauck U.S. Pat. No. 3,923,423; the Dunn U.S. Pat. No. 3,921,748; the Schiber U.S. Pat. No. 3,558,239; and the Davis U.S. Pat. No. 2,348,113.